Re: [Qemu-devel] [PATCH v11 00/28] IOMMU: Enable interrupt remapping for Intel IOMMU

[Radim Krčmář]

2016-07-05 16:19+0800, Peter Xu:
> This is v11 of Intel IR work. It is rebased to mst's branch
> "tags/for_upstream", commit:
> 
>   "278a2a2 vmw_pvscsi: remove unnecessary internal msi state flag"
> 
> This series mainly fixed several issues in v10 review comments, fixed
> one bug with RHEL guests, added acked-by for Paolo, and a fresh new
> rebase as mentioned above.
> 
> To make it fast, I only did quick tests for this versiont. But at
> least it should cover basic functions like: IOAPIC, MSI, multiple
> vcpus, different guests (4.7 upstream and rhel 7.2), vhost, split/off
> irqchips. More tests to be done.
> 
> Meanwhile, there are several pending issues to be solved, which is
> queued in my todo list and I'll continue the work after this series is
> merged.
> 
> Online branch:
> 
>   https://github.com/xzpeter/qemu vtd-intr-v11
> 
> Please review, thanks.

Testing found only one bug:

The patchset doesn't work with if you have 16 APICs with IDs 0-15 and
then some more, because KVM has hacked x2APIC support that translates
logical interrupt with destination 0xff (first cluster of 16 VCPUs) into
a broadcast (also works with lowest-priority) and then picks VCPU > 16
as the destination, leading to messages like

  do_IRQ: 17.209 No irq handler for vector

The fun part is that this is how KVM wanted to behave, when it allowed
x2apic with unremapped ioapic. :(
KVM could either finally remove x2apic without IR or add a way in which
userspace could say whether an interrupt is x2APIC or not.

QEMU cannot do anything to work around the bug, so I think it would be
best to disable EIM for now.